A critical roadblock in the development of an HIV-1 vaccine is our current inability to deliver HIV-1 antigens efficiently to the immune system and to prime predictable, high frequency immune responses in humans. Recombinant, replication-incompetent adenovirus serotype 5 (rAd5) vector-based vaccines for HIV-1 have elicited potent immune responses in preclinical studies. However, the high frequency of anti-Ad5 immunity in the developing world will likely limit the immunogenicity and clinical utility of rAd5 vaccines. We therefore propose the development of novel rAd vector-based vaccines for HIV-1. [unreadable] [unreadable] We hypothesize that rAd vector-based vaccines derived from rare Ad serotypes and engineered for improved immunogenicity will prove significantly more immunogenic than rAd5 vaccines in rhesus monkeys with anti-Ad5 immunity. We further hypothesize that the optimal rAd vaccine regimen will be a heterologous prime-boost regimen involving two different serotype vectors that are both rare in human populations, engineered for optimal immunogenicity, derived from different Ad subfamilies, and distinct from Ad5. [unreadable] [unreadable] To investigate these hypotheses, we propose the following four Specific Aims: [unreadable] 1. To compare the immunogenicity of rAd5, rAd35, and capsid chimeric rAd5/rAd35 vectors in rhesus monkeys with anti-Ad5 immunity; [unreadable] 2. To assess the immunogenicity of heterologous rAd prime-boost regimens involving vectors derived from two different Ad subfamilies in mice; [unreadable] 3. To assess the immunogenicity and protective efficacy of the optimal heterologous rAd prime-boost regimen against an SIVmac251 challenge in rhesus monkeys with and without anti-Ad5 immunity; and 4. To determine the immunogenicity and protective efficacy of the optimal heterologous rAd prime- boost regimen delivered by either systemic or mucosal routes against repetitive, low-dose, mucosal SIVmac251 challenges in rhesus monkeys with anti-Ad5 immunity. [unreadable] [unreadable] The overall goal of these studies is to develop a novel heterologous rAd prime-boost regimen for HIV-1 that is highly immunogenic in the presence of anti-Ad5 immunity and that can be advanced rapidly into clinical vaccine trials in the developing world. [unreadable] [unreadable]